The invention relates to a method of denoxing flue gases, wherein the flue gases which arise in a rotary kiln for sintering of cement clinker are guided into a calcination zone for deacidification of raw meal, aqueous ammonia solution, ammonia (NH3) or substances that release ammonia for denoxing of the flue gases by the method of selective noncatalytic reduction (SNCR) are injected into the calcination zone, and the flue gas stream together with any ammonia slip that arises in the denoxing is guided through a heat exchanger and at least one device for dedusting. The invention further relates to a corresponding plant for denoxing flue gases, having a rotary kiln for sintering of cement clinker, a calcination zone for deacidification of raw meal, preferably formed in a kiln riser duct and/or a calciner, at least one device for injection of aqueous ammonia solution, ammonia (NH3) or substances that release ammonia into the calcination zone, and a heat exchanger having at least one device for dedusting of flue gas which follows on from the heat exchanger, and a heat exchanger offgas conduit which follows on from the latter.
In the production of cement, silicate-containing and carbonate-containing raw meal is sintered in a rotary kiln to give cement clinker. At the high temperatures that are needed for this purpose in the burner flames in the rotary kiln, nitrogen oxides (NOx) form as a result of combustion of the nitrogen present in atmospheric air. A further nitrogen oxide source may be present in the fuel which is used to generate the heat in the rotary kiln. For the emission of nitrogen oxides, however, there are limits stipulated on the part of the legislator, since nitrogen oxides have a number of adverse effects on humans and the environment. More particularly, nitrogen oxides are the cause of acid rain; in addition, they contribute to degradation of ozone in the stratosphere and to global warming. In the production of cement, as in other firing units as well, it is therefore necessary to use methods of denoxing the flue gases that leave the rotary kiln in the opposite direction to the material stream. They flow into a calcination zone in which the raw meal is deacidified. The zone is most frequently formed within the kiln riser duct or within a calciner, or within a kiln riser duct and within a downstream calciner.
A first step for nitrogen oxide reduction is given by the process described in EP0854339A1, the step of conducting the feeding of the cooling zone output air and the addition of the preheated material in a staged manner, such that temperature windows favorable for the nitrogen oxide reduction exist in the calcination zone. For even greater reduction in the nitrogen oxide contents in the flue gas, it is possible to add ammonia (NH3) to the reaction space of the calcination zone. For this purpose, an aqueous ammonia solution, ammonia or a substance that releases ammonia is injected into the calcination zone or into the kiln riser duct and/or the calciner at one or more suitable points. The denoxing here is based on the known method of selective noncatalytic reduction (SNCR), in which ammonia is converted by thermolysis with the nitrogen oxides to nitrogen and water (preferably within a temperature window from 900° to 1000° C.).
The conventional SNCR process is limited by the problem that excess ammonia that does not take part in the nitrogen oxide reduction reaction, called the ammonia slip, is emitted from the plant. However, ammonia likewise has harmful effects to humans and the environment; for example, chronic respiratory pathway disorders can be caused by ammonia. It is considered to be likely that the legislator will set a limit in the near future. The more nitrogen oxides have to be reduced, the greater the slip of ammonia. In order to achieve ever further-reaching degradation of nitrogen oxides without considerable emission of ammonia, therefore, the denoxing is now also undertaken by the method of selective catalytic reduction (SCR), as described, for example in EP2444145A1. In the SCR method, the nitrogen oxides are reduced by a chemical reaction with ammonia over a catalyst, which results in nitrogen and water as products. Frequently, these catalysts consist of titanium dioxide, vanadium pentoxide and/or tungsten dioxide. Even though this method on the one hand offers very high-performance denoxing of flue gases, it has the drawback on the other hand that, in industrial use in cement production, very large catalyst volumes with several catalyst layers have to be installed. Such a technology is associated with very high capital costs and operating costs (in maintenance, repair and cleaning) because of the large catalysts. Furthermore, large catalyst volumes lead to correspondingly high pressure drops in the gas volume flow rate.
It is therefore an object of the invention to propose a method of high-performance denoxing of flue gases with only low emission of ammonia, which is suitable for use in cement production and in which it is possible to use catalysts having advantageously smaller volumes compared to conventional catalysts, especially those used in the SCR method. It is a further object of the invention to propose a plant corresponding to this method for denoxing of flue gases.